1. Field of the Invention
The present invention relates to receiver location systems in general and, more particularly, to positioning systems for mobile radio receivers in networks of transmitters.
2. Description of the Related Art
There are many systems known in the art by which the position of a receiver can be determined. One such method, when applied to the Global System for Mobiles (GSM), is known by its standardised acronym as E-OTD (Enhanced-Observed Time Difference), which uses the relative timing offsets of signals received from transmitters by a mobile terminal together with the relative timing offsets of the same signals received by a fixed receiver whose position is known. The second set of measurements by the fixed receiver is required since the transmissions may not be synchronised with respect to each other so that their relative transmission time offsets (i.e. the offsets in the times at which identical parts of the signals are transmitted from different transmitters) are constantly varying and otherwise unknown. A similar method, when applied to the Universal Mobile Telecommunication System (UMTS), is known by the acronym OTDOA (Observed Time Difference of Arrival).
Two principal, and different, methods of using the timing offsets in the position computation have been described in the art. In one e.g. EP-A-0767594, WO-A-9730360, U.S. Pat. No. 6,108,553 and AU-B-716647, the details of which are hereby incorporated by reference, the signals measured by the fixed receiver are used, in effect, to ‘synchronise’ the transmissions from the different transmitters. The instantaneous transmission time offsets of each transmitter relative to its neighbours are calculated from the values measured at the fixed receiver using the known positions of the fixed receiver and the transmitters. The timing offsets measured by the mobile station can then be used in a calculation based on well-known standard techniques in which the points of intersection of two or more hyperbolic position lines predict the position of the mobile station.
The other method, see our EP-B-0303371, WO-A-8901637, U.S. Pat. No. 6,094,168 and EP-A-1025453, WO-A-9921028 (U.S. Ser. No. 09/529,914) the details of which are hereby incorporated by reference and which refer to a system known as Cursor®, makes use of the measurements made by both the fixed receiver and the mobile station to calculate the relative time difference between the signals received from each transmitter by both receivers. This results in a calculation based on the intersection of circles centred on the transmitters.
All E-OTD systems, and OTDOA systems, require the measurement of the times of arrival of radio signals from at least three transmitters at both the mobile station (MS) and at least one other receiver, each radio receiver having its own internal clock. In the GSM system, the transmitters are known as Base Transceiver Stations (BTSs) and the fixed receivers are known as Location Measurement Units (LMUs). The signals from each BTS within range of the MS are received both by the MS itself and by one or more LMUs. Our applications WO-A-9921028 (U.S. Ser. No. 09/529,914) and WO-A-0073813 (U.S. Ser. No. 09/830,447), the contents of each of which are hereby incorporated by reference, describe how the measurements made by a network of LMUs may be combined together to provide a list of the measurements which would have been produced by a single LMU, the virtual LMU or VLMU, at a given location which could have received the signals from all the transmitters.
In these (and other similar) systems, measurement of the time of arrival of a signal from a transmitter is key. In real radio systems many copies of the same transmitted signal arrive at the receiver, each having traversed a different path and arriving with a different time delay; this is known as multipath propagation. Making timing measurements in the presence of multipath signals can be problematic because of the receipt of plural signal copies. The shortest path between a transmitter and a receiver is the line of sight (LOS) path. In practice the radio channel, and hence the radio path, between the transmitter and the receiver is complicated because of reflection, refraction and diffraction of signals between transmission and reception.
An example of both signal reflection and refraction by buildings is shown in FIG. 1, which depicts three MSs 101, 102 and 103, a transmitter 104 and buildings 105 and 106. In this case each MS receives only one copy of the transmitted signal. In most practical cases the signal not only propagates along a direct LOS path, e.g. to MS 101, but encounters obstacles such as buildings 105 and 106 causing additional reflected paths, such as that shown to MS 102, or refracted paths, such as that shown to MS 103. The reflected and refracted signal paths are always longer than the LOS path and so arrive later. If the arrival time, or arrival time relative to a reference of an appropriate type, of a transmitted signal is to be used to compute the position of the receiver, it can be important to use the earliest arriving copy, preferably the LOS copy. The first arriving signal at an MS may not necessarily be the LOS signal if there is obstruction between the transmitter and the receiver such that there is no LOS signal.
A vertically oriented antenna may be directional or omni-directional in the horizontal plane. FIG. 2a depicts a model of a directional antenna 201 in plan view, the plane of the page representing the antenna's horizontal plane. Antenna 201 transmits into a sector, which is modelled by the direction 202 of the centre of the sector (usually in degrees clockwise from north) and the half power angle 203 of the sector. This model is a simplification of the observed horizontal radiation pattern of a real directional antenna, an example of which is shown in FIG. 2b. The simplified model has the back-lobe 204 and side-lobes 205 removed. Hereafter the simplified model of a directional transmitter depicted in FIG. 2a will be referred to, but any complications arising from the simplifications will be discussed when they occur.
A cell of a cellular communications network corresponds to the area served by one antenna. Where the antenna has an omni-directional pattern it is situated at the centre of the cell. However, it is common to co-locate several antennas on the same site, and in this case the antennas are directional, each covering separate adjacent cells. For example, in GSM, several directional antennae (usually three) can be situated at the same site, each transmitting on a different frequency in a specified direction with a specified beam width. A signal transmitted entirely away from an MS clearly cannot be received by that MS along a LOS path. Therefore the use of the relative time of arrival of that signal in the computation of the position of the MS would lead to error. (In practise, there is always a small probability that the signal received in such circumstances has come directly from a side- or back-lobe of the transmitting antenna.) A more accurate position of the MS is obtainable using the method of the invention, which takes into account the directionality of some of the transmitters in a cellular network.